The NMDA subtype of glutamate-gated ion channels mediates excitatory synaptic transmission between neurons in the central nervous system (Dingledine et al. (1999), Pharmacological Reviews 51:7-61). Animal models of stroke and brain trauma confirm that glutamate released from affected neurons can overstimulate N-methyl-D-aspartate (NMDA) receptors, which in turn causes neuronal death. Therefore, compounds that block NMDA receptors have been considered candidates for treatment of stroke and head injuries.
NMDA receptors are composed of NR1, NR2 (A, B, C, and D), and NR3 (A and B) subunits, which determine the functional properties of native NMDA receptors. Expression of the NR1 subunit alone does not produce a functional receptor. Co-expression of one or more NR2 subunits is required to form functional channels. In addition to glutamate, the NMDA receptor requires the binding of a co-agonist, glycine, to allow the receptor to function. A glycine binding site is found on the NR1 and NR3 subunits, whereas the glutamate binding site is found on NR2 subunits. At resting membrane potentials, NMDA receptors are largely inactive due to a voltage-dependent block of the channel pore by magnesium ions. Depolarization releases this channel block and permits passage of calcium as well as other ions.
The NMDA receptor is modulated by a number of endogenous and exogenous compounds including, sodium, potassium and calcium ions that can not only pass through the NMDA receptor channel but also modulate the activity of receptors. Zinc blocks the channel through NR2A- and NR2B-containing receptors noncompetitive and voltage-independent manner. Polyamines can also either potentiate or inhibit glutamate-mediated responses.
Stroke is the third leading cause of death in the United States and the most common cause of adult disability. In an ischemic stroke, which is the cause of approximately 80% of strokes, a blood vessel becomes occluded and the blood supply to part of the brain is blocked. Ischemic stroke is commonly divided into thrombotic stroke, embolic stroke, systemic hypoperfusion (Watershed or Border Zone stroke), or venous thrombosis. NMDA antagonists have been studied as neuroprotective agents for acute stroke. However, these agents, including Dextrorphan, Selfotel and aptiganel HCl (Cerestat) all showed certain toxicity profiles that required halting trials of these agents. Early clinical studies suggest that psychomimetic side effects occur less frequently with glycine site NMDA antagonists, however clinical studies have not supported a protective role for these agents (http://www.emedicine.com/neuro/topic488.htm, Lutsep & Clark “Neuroprotective Agents in Stroke”, Apr. 30, 2004).
Epilepsy has also long been considered a potential therapeutic target for glutamate receptor antagonists. NMDA receptor antagonists are known to be anti-convulsant in many experimental models of epilepsy (Bradford (1995) Progress in Neurobiology 47:477-511; McNamara, J. O. (2001) Drugs effective in the therapy of the epilepsies. In Goodman & Gliman's: The pharmacological basis of therapeutics [Eds. J. G. Hardman and L. E. Limbird] McGraw Hill, New York).
NMDA receptor antagonists may also be beneficial in the treatment of chronic pain. Chronic pain, including neuropathic pain such as that due to injury of peripheral or central nerves, has often proved very difficult to treat. Treatment of chronic pain with ketamine and amantadine has proven beneficial, and it is believed that the analgesic effects of ketamine and amantadine are mediated by block of NMDA receptors. Several case reports have indicated that systemic administration of amantadine or ketamine substantially reduces the intensity of trauma-induced neuropathic pain. Small-scale double blind, randomized clinical trials corroborated that amantadine could significantly reduce neuropathic pain in cancer patients (Pud et al. (1998), Pain 75:349-354) and ketamine could reduce pain in patients with peripheral nerve injury (Felsby et al. (1996), Pain 64:283-291), peripheral vascular disease (Perrson et al. (1998), Acta Anaesthesiol Scand 42:750-758), or kidney donors (Stubhaug et al. (1997), Acta Anaesthesiol Scand 41:1124-1132). “Wind-up pain” produced by repeated pinpricking was also dramatically reduced. These findings suggest that central sensitization caused by nociceptive inputs can be prevented by administration of NMDA receptor antagonists.
NMDA receptor antagonists can also be beneficial in the treatment of Parkinson's Disease (Blandini and Greenamyre (1998), Fundam Clin Pharmacol 12:4-12). The anti-Parkinsonian drug, amantadine, is an NMDA receptor channel blocker (Blanpied et al. (1997), J Neurophys 77:309-323). In a small clinical trial, Amantadine reduced the severity of dyskinesias by 60% without reducing the antiparkinsonian effect of L-DOPA (Verhagen Metman et al. (1998), Neurology 50:1323-1326). Likewise, another NMDA receptor antagonist, CP-101,606, potentiated the relief of Parkinson's symptoms by L-DOPA in a monkey model (Steece-Collier et al., (2000) Exper. Neurol., 163:239-243).
NMDA receptor antagonists may in addition be beneficial in the treatment of brain cancers. Rapidly-growing brain gliomas can kill adjacent neurons by secreting glutamate and overactivating NMDA receptors such that the dying neurons make room for the growing tumor, and may release cellular components that stimulate tumor growth. Studies show NMDA receptor antagonists can reduce the rate of tumor growth in vivo as well as in some in vitro models (Takano, T., et al. (2001), Nature Medicine 7:1010-1015; Rothstein, J. D. and Bren, H. (2001) Nature Medicine 7:994-995; Rzeski, W., et al. (2001), Proc. Nat'l Acad. Sci. 98:6372).
While NMDA-receptor antagonists might be useful to treat a number of very challenging disorders, to date, dose-limiting side effects have prevented clinical use of NMDA receptor antagonists for these conditions. Thus, despite the tremendous potential for glutamate antagonists to treat many serious diseases, the severity of the side effects have caused many to abandon hope that a well-tolerated NMDA receptor antagonist could be developed (Hoyte L. et al (2004) “The Rise and Fall of NMDA Antagonists for Ischemic Stroke Current Molecular Medicine” 4(2): 131-136; Muir, K. W. and Lees, K. R. (1995) Stroke 26:503-513; Herrling, P. L., ed. (1997) “Excitatory amino acid clinical results with antagonists” Academic Press; Parsons et al. (1998) Drug News Perspective II: 523 569).
pH Sensitive NMDA Receptors
Two of the most prevalent subtypes of NMDA receptors (including the NR2A and NR2B subunits or an alternatively spliced NR1 subunit) have the unusual property of being normally inhibited by protons by about 50% at physiological pH (Traynelis, S. F. and Cull-Candy, S. G. (1990) Nature 345:347; Traynelis et al. (1995) Science 268: 873-876; Traynelis et al. (1998), J Neurosci 18:6163-6175).
The extracellular pH is highly dynamic in mammalian brain, and influences the function of a multitude of biochemical processes and proteins, including glutamate receptor function. The pH-sensitivity of the NMDA receptor has received increasing attention for at least two reasons. First, the IC50 value for proton inhibition of pH 7.4 places the receptor under tonic inhibition at physiological pH. Second, pH changes are extensively documented in the central nervous system during synaptic transmission, glutamate receptor activation, glutamate receptor uptake, and prominently during pathological states such as ischemia and seizures (Siesjo, B K (1985), Progr Brain Res 63:121-154; Chesler, M (1990), Prog Neurobiol 34:401-427; Chesler and Kaila (1992), Trends Neurosci 15:396-402; Amato et al. (1994), J Neurophysiol 72:1686-1696).
During stroke, transient ischemia leads to a dramatic drop of pH to 6.4-6.5 in the core region of the infarct, with a modest drop in regions surrounding the core. The penumbral region, which surrounds the core and extends outward, suffers significant neuronal loss. The pH in this region drops to around pH 6.9. The pH-induced drops are exaggerated in presence of excess glutamate, and attenuated in hypoglycemic condition (see, for example, Mutch & Hansen (1984) J Cereb Blood Flow Metab 4: 17-27, Smith et al. (1986) J Cereb Blood Flow Metab 6: 574-583; Nedergaard et al. (1991) Am J Physiol 260 (Pt3): R581-588; Katsura et al (1992a) Euro J Neursci 4: 166-176; and Katsura & Siesjo (1998) “Acid base metabolism in ischemia” in pH and Brain function (Eds Kaila & Ransom) Wiley-Liss, New York).
In addition to ischemia, there are various other examples of conditions in which pH changes can be associated with pathological processes, including neuropathic pain, Parkinsons disease, epilepsy and traumatic brain injuries.
Neuropathic pain, which is due to hyperactivity of nerve fibers in the dorsal horn of the spinal chord can be associated with pH changes in the spinal cord. Single electrical stimulation of isolated spinal cord from rat pups produces an alkaline shift of 0.05 pH units, and a 0.1 pH unit shift following 10 Hz stimulation which is followed by acidification after cessation of the stimuli. This acidification is greater in older animals (Jendelova & Sykova (1991) Glia 4: 56-63), indicating an increased pH differential underlying the stimuli. Similarly, 30-40 Hz stimulation of the dorsal root in frog produced in vivo a transient extracellular acidification reaching a maximum ceiling of 0.25 pH unit reduction in the lower dorsal horn. Extracellular pH changes increased with stimulus intensity and frequency (Chvatal et al. (1988) Physiol Bohemoslov 37: 203-212). Further, high frequency (10-100 Hz) nerve stimulation in adult rat spinal cord in vivo produced triphasic alkaline-acid-alkaline shifts in extracellular pH (Sykova et al. (1992) Can J Physiol Pharmacol 70: Suppl 5301-309). Additionally, it has been shown that acute nociceptive stimuli (pinch, press, heat) applied to the rat hindpaw produced transient acidification of 0.01-0.05 pH units in the lower dorsal horn in vivo (laminae III-VII). Chemical or thermal peripheral injury produced prolonged 2 hour decreases in interstitial pH of 0.05-0.1 pH units. High frequency nerve stimulation produced an alkaline pH shift followed by a dominating 0.2 pH unit acid shift (Sykova & Svoboda (1990) Brain Res 512: 181-189). Thus, increased firing of pain fibers can cause a decrease in pH (acidification) of the dorsal horn of the spinal cord.
Subthalamic neurons are overactive in Parkinson's disease, which may result in a lower local pH. There is a correlation in brain regions between neuronal activity and extracellular pH, with activity causing acidification. High frequency stimulation of brain slices gives an initial acidification followed by an alkalinization, followed by a slow acidification (See, for example, Chesler (1990) Prog Neurobiol 34: 401-427, Chesler & Kaila (1992) Tr Neurosci 15: 396-402, and Kaila & Chesler (1998) “Activity evoked changes in extracellular pH” in pH and Brain function (eds Kaila and Ransom). Wiley-Liss, New York).
Acidification also occurs during seizures. Electrographic seizures in a wide range of preparations have been shown to cause a change in extracellular pH. For example, up to a 0.2-0.36 drop in pH can occur in cat fascia dentata or rat hippocampal CA1 or dentate during an electrically or chemically evoked seizure. Deeper drops in pH approaching 0.5 can occur under hypoxic conditions (Siesjo et al (1985) J Cereb Blood Flow Metab 5: 47-57; Balestrino & Somjen (1988) J Physiol 396: 247-266; and Xiong & Stringer (2000) J Neurophysiol 83: 3519-3524).
In addition, other types of brain injury can result in acidification. “Spreading depression” is a term used to describe a slowly moving wave of electrical inactivity that occurs following a number of traumatic insults to brain tissue. Spreading depression can occur during a concussion or migraine. Acidic pH changes occur with spreading depression. Systemic alkalosis can occur with reduction in overall carbon dioxide content (hypocapnia) through, for example, hyperventilation. Conversely, systemic acidosis can occur with an increase in blood carbon dioxide (hypercapnia) during respiratory distress or conditions that impair gas exchange or lung function. Diabetic ketoacidosis and lactic acidosis represent three of the most serious acute complications of diabetes and can result in brain acidification. Further, fetal asphyxia during parturition occurs in 25 per 1000 births at term. It involves hypoxia and brain damage that is similar but not identical to ischemia.
The acidification associated with pathological situations can partially inhibit NMDA receptors, which provides negative feedback that reduces their contribution to neurotoxicity and seizure maintenance (Kaku et al. (1993), Science 260:1516-1518; Munir and McGonigle (1995), J Neurosci 15:7847-7860; Vornov et al. (1996), J Neurochem 67:2379-2389; Gray et al. (1997), J Neurosurg Anesthesiol 9:180-187; O'Donnell and Bickler (1994), Stroke 25:171-177; reviewed by Tombaugh and Sapolsky (1993), J Neurochem 61:793-803; (Balestrino and Somjen (1988), J Physiol (Lond) 396:247-266; Velisek et al. (1994), Exp Brain Res 101:44-52). However, the pH sensitivity of glutamate transporters increases the likelihood that extracellular glutamate levels will be high during a period of acidification (Billups and Attwell (1996), Nature (Lond) 379:171-173), which enhances the opportunity for post-insult treatment of, for example, stroke with NMDA receptor antagonists (Tombaugh and Sapolsky (1993), J Neurochem 61:793-803).
Until 1995, it was not known whether the proton-sensitive property of the NMDA receptor could be exploited as a target for small molecule modulation of the receptor to develop therapeutics. Traynelis et al. (1995 Science 268:873) reported for the first time that the small molecule spermine could modulate NMDA receptor function through relief of proton inhibition. Spermine, a polyamine, shifts the pKa of the proton sensor to acidic values, reducing the degree of tonic inhibition at physiological pH, which appears as a potentiation of function (Traynelis et al. (1995), Science 268:873-876; Kumamoto, E (1996), Magnes Res 9(4):317-327).
In 1998, it was determined that the mechanism of action of the phenylethanolamine NMDA antagonists involved the proton sensor. Ifenprodil and CP-101,606 increased the sensitivity of the receptor to protons, thereby enhancing the proton inhibition. By shifting the pKa for proton block of NMDA receptors to more alkaline values, ifenprodil binding causes a larger fraction of receptors to be protonated at physiological pH and, thus, inhibited. In addition, ifenprodil was found to be more potent at lower pH (6.5) than higher pH (7.5) as tested in an in vitro model of NMDA-induced excitotoxicity in primary cultures of rat cerebral cortex (Mott et al. 1998 Nature Neuroscience 1:659). These compounds have exhibited neuroprotective properties in preclinical models and lack the severe side-effect liability of other types of NMDA antagonists (e.g. PCP-like psychotic symptoms and cardiovascular effects). Other NMDA receptor-selective derivatives of ifenprodil are being considered for clinical development, including CP101,606 (Menniti et al. (1997), Eur J Pharmacol 331:117-126), Ro 25-6981 (Fischer et al. (1997), J Pharmacol Exp Ther 283:1285-1292) and Ro 8-4304 (Kew et al. (1998), Br J Pharmacol 123:463-472). Unfortunately, ifenprodil and several of its analogs, including eliprodil and haloperidol (Lynch and Gallagher (1996), J Pharmacol Exp Ther 279:154-161; Brimecombe et al. (1998), Pharmacol Exp Ther 286(2):627-634), block certain serotonin receptors and calcium channels in addition to NMDA receptors, limiting their clinical usefulness (Fletcher et al. (1995), Br J Pharmacol 116(7):2791-2800; McCool and Lovinger (1995), Neuropharmacology 34:621-629; Barann et al. (1998), Naunyn Schmiedebergs Arch Pharmacol 358:145-152).
WO 02/072542 to Emory University describes a class of pH-dependent NMDA receptor antagonists that exhibit pH sensitivity tested in vitro using an oocyte assay and in an experimental model of epilepsy.
WO 06/023957 to Emory University describes processes for selection of a compound which may be useful in the treatment of an ischemic injury or a disorder that lowers the pH in a manner that activates the NMDA receptor antagonist.
There remains a need for improved neuroprotective compounds and methods for the treatment of neuropathologies that have reduced toxicity. In particular there is a need for improved treatments for neuropathic pain, inflammatory pain, stroke, traumatic brain injury, global ischemia, hypoxia, spinal cord trauma, epilepsy, and other neurodegenerative diseases and disorders.
It is therefore an object of the present invention to provide new compounds, pharmaceutical compositions and methods for the treatment of neuropathic and neurodegenerative diseases and disorders.